Port counts (the number of ports used for connection by nodes provided to a transmission channel) is increasing as the scale of network devices used for optical communication increases, and per-port transmission speeds are also increasing. The inverse MUX transmission scheme is known as a transmission method that uses such high-speed lines to perform optical communication over long distances with high adaptability. Inverse MUX transmission is also referred to as “inverse multiplexing transmission.”
In an inverse MUX scheme, a high-speed line having a high transmission speed is separated (divided) into a plurality of low-speed lines, and the separate low-speed lines are then multiplexed into a high-speed line. This scheme has advantages in that degradation of the optical signal that occurs in a high-speed line due to the use of low-speed lines can be suppressed when transmission in the high-speed line is made difficult by light loss and other optical fiber loss.
In FIG. 33, a high-speed optical signal transmitted from a device (not shown) in a prior stage is presented to a transmission node (terminal node) 10 and converted to an electrical signal in a high-speed optical transceiver 12. A plurality of low-speed electrical signals separated in a separation circuit 14 is converted to optical signals by low-speed optical transceivers 18 that constitute a WDM transmission device 16, and the optical signals are transmitted via optical fibers or other optical transmission channels 20. The optical signals transmitted from the transmission node 10 are again converted to electrical signals by low-speed optical transceivers 24 that constitute a WDM transmission device 22 in a reception node (terminal node) 25, the skew (arrival time difference or delay difference) that occurs during transmission is compensated for in a de-skew circuit 26, and the electrical signals are multiplexed in a multiplexing circuit 28. The signal is then converted to an optical signal by a high-speed optical transceiver 30 and transmitted to a high-speed optical transmission channel 31.
However, in the conventional technique described in Patent Document 1, regardless of the scheme employed for separating the high-speed signal, the skew (amount) that occurs between the plurality of low-speed optical fibers increases in proportion to the transmission distance due to the effects of wavelength dispersion in the optical fibers that are the transmission channels when a single high-speed line is divided into a plurality of low-speed lines for transmission. Consequently, a large capacity of buffer memory is required in the terminal node (reception node) that reproduces the high-speed signal. Alternatively, when the high-speed signal is separated into relatively small frames and transmitted, the allowable skew for small frames decreases in size, and the transmission distance therefore cannot be increased.
In order to overcome these problems, a plurality of relay nodes, e.g., two relay nodes ND1 and ND2, may be added between the WDM transmission devices 16 and 22 in FIG. 33, for example, as shown in FIG. 34. Such a configuration enables reproduction and relaying of electrical signals in the optical transceivers 40, 42, 44, 46 that constitute the WDM transmission devices 32, 34, 36, 38, respectively, and it is therefore possible to prevent the problem of degradation of the optical signal waveforms. However, since there is no function for skew compensation between the nodes, the skew increases according to the transmission distance, and it is sometimes impossible to compensate for the skew in the reception node 25 for terminating the high-speed frames.
Since skew compensation is made possible by physical compensation of dispersion values using dispersion-compensated fiber or the like, the problems described above can be overcome. However, a high-performance transmission system must be designed in this method, and the additional problem of high cost therefore occurs.
Optical communication devices have been developed in the past that have cross-connection capability and can accommodate client lines (see Patent Document 1, for example). FIG. 35 is a block diagram showing the conventional optical communication device disclosed in Patent Document 2. As shown in FIG. 35, this conventional optical communication device is provided with a switch card in which a time division switch and a spatial switch are combined. Specifically, in this conventional optical transmission device, a switch card 101 is provided, and a spatial switch 102 and two time division switches 103 connected to the spatial switch 102 are provided to the switch card 101. The line speed of the spatial switch 102 is 10 gigabits per second (hereinafter indicated as 10 Gb/s).
The spatial switch 102 is connected to two line cards 105 on the transmission channel side via an NNI (Network Node Interface) 104. The line cards 105 transmit high-speed traffic. Each line card 105 is provided with a plurality of 10 Gb/s lines 106 that is standardized as high-speed lines, and the 10 Gb/s lines 106 have one-to-one connections to the NNI 104.
The time division switches 103 are connected to a line card 108 on the line-out side via a UNI (User Network Interface) 107. The line card 108 transmits low-speed traffic. A plurality of 2.5 Gb/s lines 109 and 600 megabit per second (hereinafter indicated as 600 Mb/s) lines 110 is provided to the line card 108, and the lines are connected to the UNI 107.
The line speed of the low-speed traffic inputted from the line card 108 to the time division switches 103 via the UNI 107 is boosted to the line speed handled by the spatial switch 102, i.e., 10 Gb/s, by the multiplexing function provided to the time division switches 103. The line speed of the high-speed traffic transmitted through the 10 Gb/s lines 106 that are connected to the line card 105 is also equal to the line speed handled by the spatial switch 102. The line cards 105 in which the high-speed lines are mounted, and the spatial switch 102 in the switch card 101 are thereby directly connected via the NNI. As a result, the traffic of the high-speed lines and the signal in which the plurality of UNI traffic units is multiplexed can be switched in unified fashion by the spatial switch, and an optical communication device is created that has a simple overall structure.
However, the optical communication device disclosed in Patent Document 2 has such problems as the following. A first problem is that since the functions for increasing the flexibility of the device are integrated in the switch card portion, the load of the switch card increases, and the power consumption, size, and cost of the switch card increase. Relatively small size and low cost can be obtained when the spatial switch that is mounted in the switch card is limited to a specific line capacity. However, the optical communication device disclosed in Patent Document 1 uses a time division switch in order to efficiently multiplex and accommodate a plurality of client-side lines having different types and speeds. The switch card portion therefore requires a functional circuit for high-load frame processing, which results in increased power consumption. This makes it difficult to increase the density and expandability of the switch card.
As a second problem, when a mismatch exists between the speed of the NNI and the signal speed that can be processed by the spatial switch, another circuit is required to compensate for the mismatch, and the structure of the switch card therefore becomes more complex. In Patent Document 1, the speed of the spatial switch is the common speed of 10 Gb/s as the line speed of the high-speed lines connected to the NNI, but switches that are capable of operating at a line speed of 10 Gb/s are limited to small-scale electric switches or optical switches. The limitation on the line speed that can be handled by an electrical spatial switch is also documented in patent document 1. The line speed of 10 Gb/s is not only a problem of device technology for an electrical switch, but also of electrical transmission technology. There is a large amount of waveform distortion due to high transmission loss of high-frequency components at 10 Gb/s, and the transmission distance is limited to within a few centimeters unless a high degree of compensation is applied, and it is difficult to directly connect the switch to an optical transceiver or the like.
When an optical switch is used in order to process a 10 Gb/s signal, electrical/optical conversion is required prior to introduction to an optical switch, and prior to introduction of a signal inputted via the NNI to an optical switch after the signal inputted via the UNI is multiplexed. Conversely, optical/electrical conversion is necessary when a signal is conducted from an optical switch to the UNI and the NNI. As a result, the need arises to provide the switch card with an electrical/optical conversion device and an optical/electrical conversion device, which even further increases the complexity and size of the switch card.
When a line-side optical signal is introduced to an optical switch without creating an electrical termination, the cost, size, and complexity of the optical transmission function increase due to the provision of an optical transmission device for compensating for wavelength dispersion and light loss in the area of the optical switch. Unless there is a wavelength conversion device, there is no connectivity between channels in a DWDM (Dense Wavelength Division Multiplexing) signal, and the number of connectible ports is limited. Flexibility therefore decreases.
Patent Document 3 therefore discloses a technique in which the signal of a high-speed line is converted to frames by a reception circuit, and then divided into a plurality of blocks and introduced to a spatial switch, rather than being introduced to the spatial switch as an unmodified unit. FIG. 36 is a block diagram showing the spatial switch and the area around the spatial switch of the conventional optical communication device disclosed in Patent Document 3. In this conventional optical communication device as shown in FIG. 36, a spatial switch 201 is provided, four VC-4 separation units 202 are connected to the reception side of the transmission channels of the spatial switch 201, and four VC-4 multiplexers 203 are connected to the transmission side. An STM-16 reception interface 204 is connected to each VC-4 separation unit 202, and an STM-16 transmission interface 205 is connected to each VC-4 multiplexer 203. A single VC-4 separation unit 206 and a single VC-4 multiplexer 207 are connected to the subscriber side (client side) of the spatial switch 201, an STM-4 reception interface 208 is connected to the VC-4 separation unit 206, and an STM transmission interface 209 is connected to the VC-4 multiplexer 207.
In the optical communication device thus configured as disclosed in Patent Document 3, the signals inputted from the transmission channels to the STM-16 reception interfaces 204 are converted to frames by the STM-16 reception interfaces 204, and then divided into 150 Mb/s VC-4 blocks in the VC-4 separation units 202 and inputted to the optical switch 201. The signals outputted to the transmission channel side from the optical switch 201 are multiplexed in the VC-4 multiplexers 203 and are outputted via the STM-16 transmission interfaces 205. The signals inputted from the subscriber side are divided into frames by the STM-4 reception interfaces 208, divided into 150 Mb/s blocks by the VC-4 separation units 206, and inputted to the optical switch 201. The signals outputted from the optical switch 201 to the subscriber side are multiplexed in the VC-4 multiplexers 207, and outputted via the STM-4 transmission interface 209.
In this optical communication device, the signals of the high-speed line are introduced to the spatial switch after being divided into 150 Mb/s blocks rather than being introduced to the spatial switch as an unmodified unit. A primary cross connect is thereby performed between the high-speed line and the low-speed line by 150 Mb/s blocks. The signal is furthermore inputted to a time division switch (not shown), and a cross connect in smaller units is performed in order to further divide and handle the 150 Mb/s blocks on the low-speed line side. The entire circuit structure is thus simplified in Patent Document 3 by separating the switch into the levels of spatial switch and time division switch. The signal is introduced to the spatial switch after the high-speed line is separated at a speed at which electrical processing is easily performed.
Patent Document 1: Japanese Laid-open Patent Application No. 2002-135223
Patent Document 2: Japanese Laid-open Patent Application No. 2003-169355
Patent Document 3: Japanese Laid-open Patent Application No. 2003-061171